Carburized component and method of manufacturing the same

ABSTRACT

This invention aims to provide a carburized component realizing a larger strength for power transmission components such as gears, and a method of manufacturing the same. The carburized component of this invention, aimed at realizing the object, consists essentially of, in % by mass and both ends inclusive, C: 0.1-0.30%, Si: 0.80-1.50%, Mn: 0.30-1.20%, Cr: 2.0-5.5%, and the balance of Fe and inevitable impurities; has a mean C concentration over the range from the surface of the steel to a depth of 0.2 mm after vacuum carburization of 1.2% or more and 3.0% or less, and has a ratio of a carbide area over the range from the surface to a depth of 50 μm of 15% or more and 60% or less, has the carbide precipitated in a finely dispersed manner so that the carbide having a grain size of 10 μm or less accounts for 90% or more of the entire portion, and has a depth of a grain boundary oxide layer below the surface of 1 μm or less.

FIELD OF THE INVENTION

This invention relates to a carburized component and a method ofmanufacturing the same.

BACKGROUND ART

Gears used as power transmission components for automobiles and so forthare components suffering from dedendum fractures which occur at thededendum where bending stress happens, and from slip-induced fracture(pitting) which occurs in the vicinity of the pitch point. A techniqueof carburizing the surface of the component has, therefore, widely beenused for the purpose of fulfilling characteristics enough forwithstanding harsh conditions, and further improvement has been made bycombining various materials and heat treatments.

Particularly in recent years, a successful development has been made ona material capable of suppressing growth of grain boundary oxide layerand abnormally carburized layer during carburization, which areunderstood as being harmful to dedendum fracture. Another achievementhas been made on improvement in the strength typically by shot peening.

On the other hand, pitting has also extensively been investigated, andit has been found out that prevention of softening of the material iseffective to improve the strength. Gears cause slippage on the toothsurface thereof, and the repetitive contact generates heat at theportion just under the tooth surface. Temperature in this state is knownto fall in the range of about 200 to 300° C., and the heat generatedherein supposedly softens the material and consequently results inpitting fracture. It is therefore believed that prevention of softeningin a temperature range of about 200 to 300° C. is effective forimproving the pitting fracture, and development has been made onmaterials added with Si, Cr, Mo and so forth as alloy elements excellentin the softening resistance in this temperature range.

[Patent Document] Japanese Laid-Open Patent Publication “Tokkaihei” No.6-158266

The gear has, however, has been demanded to have a larger hardness withthe recent increases in the output of automobiles and so forth, but thepresent situation is that the above-described material is insufficientfor fulfilling the requirements.

This invention was conceived after considering the above-describedsituation, and an object thereof is to provide a carburized componentrealizing higher strength for power transmission components such asgears, and a method of fabricating said components.

SUMMARY OF THE INVENTION

Aiming at solving the aforementioned problems, a carburized componentconsisting essentially of, in percentages by mass and both endsinclusive, C: 0.1-0.30%, Si: 0.80-1.50%, Mn: 0.30-1.20%, Cr: 2.0-5.5%,and the balance between Fe and inevitable impurities;

-   -   has a mean C concentration over the range from the surface of        steel to a depth of 0.2 mm after vacuum carburization of 1.2% or        more and 3.0% or less, has a ratio of carbide area over the        range from the surface to a depth of 50 μm of 15% or more and        60% or less, has the carbide precipitated in a finely dispersed        manner so that the carbide having a grain size of 10 μm or less        accounts for 90% of the entire portion, and has a depth of a        grain boundary oxide layer below the surface of 1 μm or less.

It is also allowable to further add either or both of Mo: 0.2 to 1.0%and V: 0.2 to 1.0%.

This invention has basic features as described below. That is, a largeamount of fine carbide grains are allowed to precipitate in thesurficial portion of the component by high-concentration vacuumcarburization, and to substantially exclude the surficial grain boundaryoxide layer, to thereby raise the surface hardness and strength. Inaddition, the temper softening resistance in the temperature range fromabout 200 to 300° C. is enhanced by introducing a large amount of Si,which is realized by the vacuum carburization, and thereby a desirablelevel of surface fatigue strength can be obtained. These features can beobtained only under the appropriately-adjusted ingredients andconditions as detailed below.

C: 0.10 to 0.30%

C is an essential element for ensuring a necessary level of strength forthe component, and is necessary to contain an amount of 0.10% or more.On the other hand, an excessively large content thereof increases thehardness of the material, thus degrading the machinability, and therebymaking the machining of the component difficult. The upper limit istherefore adjusted to 0.30%.

Si: 0.80 to 1.50%

Si is an element to be contained as a deoxidizing element acting in theprocess of melting and plays an important role in this invention. Theelement dissolves into the solid matrix to thereby raise the tempersoftening resistance, so that a high level of surface fatigue strengthcan be obtained. The element can also suppress growth of coarse carbidegrains, because it shows only a small solid solubility into the carbideand raises the Si concentration in the base metal. Moreover, underprecipitation of a large amount of carbide, Si showing only a smallsolid solubility into the carbide concentrates in the matrix, andfurther improves the temper softening resistance of the matrix. Theelement is necessarily contained to an amount of 0.80% or more in orderto obtain this effect. On the other hand, an excessive content of theelement inhibits precipitation and the carburization surface reaction ofthe carbide which thereby distinctively degrades the carburizationproperty, and also degrades the ductility, which thereby makes crackingmore likely to occur in the process of plastic working. The upper limitof the content is therefore limited to 1.50%.

Si is an element promoting oxidation of the grain boundary in theprocess of general gas carburization, and the grain boundary oxidationlayer is causative of lowering the impact strength and fatigue strengthof dedendum. The gas carburization therefore cannot add a large amountof Si, whereas the vacuum carburization as described in the above canclear the problem of grain boundary oxidation, and make it possible toobtain a high-Si-content component.

Mn: 0.30 to 1.20%

Mn is an element to be contained as a deoxidizing element acting in theprocess of melting, and has an effect of improving the hardeningproperty, so that it is necessary to contain an amount of 0.30% or more.In this invention, elements having an effect of improving the hardeningproperty, such as Cr, are to be concomitantly contained, wherein theelements such as Cr, capable of forming the carbide, may sometimesresult in only an insufficient hardening property even under a raised Crcontent or the like, depending on carbide content. It is thereforeeffective to adjust the Mn content in order to obtain a necessary levelof hardening property. On the other hand, an excessive content degradesthe machinability due to an increase in the hardness of the material,thus the upper limit is adjusted to 1.20%.

Cr: 2.0 to 5.5%

Cr is an element playing an important role in this invention. This isnecessary to contain an amount of 2.0% or more, as a carbide-formingelement and as an element improving the hardening property. On the otherhand, an excessive content of the element degrades the machinability dueto increased hardness of the material, and makes a network-structuredcarbide more likely to be generated in the grain boundary. The upperlimit of the content is therefore limited to 5.5%.

Mo: 0.2 to 1.0%

Mo binds with C, similarly to Cr, to produce the carbide, and has aneffect of improving the pitting strength by raising the softeningresistance over the temperature range from 200° C. to 300° C. Theelement is preferably contained to an amount of 0.2% or more, for thepurpose of obtaining these effects. On the other hand, an excessivecontent of the element degrades the machinability due to an increase inhardness of the material, and increases the material cost. The upperlimit of the content is, therefore, preferably limited to 1.0%.

V: 0.2 to 1.0%

V binds with C, similarly to Cr and Mo, to produce the carbide, and hasan effect of improving the pitting strength by raising the softeningresistance, through production of an MC-type carbide. The element ispreferably contained to an amount of 0.2% or more, for the purpose ofobtaining these effects. On the other hand, an excessive content of theelement degrades the machinability due to an increase in hardness of thematerial. The upper limit of the content is, therefore, preferablylimited to 1.0%.

Carburization: Vacuum Carburization (at 1,000 Pa or Below)

The carburized component of this invention is subjected to vacuumcarburization. The vacuum carburization makes it possible to decreasethe growth of the grain boundary oxide layer, and is thereforesuccessful in raising the strength of the carburized component.

As described in the above, Si is added as an essential ingredient. Si isan element promoting the grain boundary oxidation in the process of thegeneral gas carburization, and such grain boundary oxidation iscausative of reducing the impact strength and fatigue strength of thededendum. It is, therefore, extremely difficult for the general gascarburization to achieve a large Si content. Whereas the vacuumcarburization can, however, suppress formation of the grain boundaryoxide layer, and can readily realize a high Si content.

Depth of Grain Boundary Oxide Layer: 1 μm or Less

The grain boundary oxide layer causes lowering in the fatigue strengthand anti-pitting strength, wherein the degree of the lowering becomeslarger as the depth increases. For the carburized component of thisinvention, the depth of grain boundary layer from the surface of thesteel after the vacuum carburization is adjusted to 1 μm or less.

Mean C Concentration Up to Depth of 0.2 mm from the Surface: 1.2% ormore and 3.0% or Less

The general carburization is normally carried out as an eutecticcarburization of the surface of steel, targeted at an eutectic C contentof 0.8%. In contrast, this invention is aimed at improving theanti-pitting property through precipitation of the carbide in thesurficial layer of the steel to thereby enhance the softeningresistance, so that it is necessary to contain C to an amount of theeutectic C content (0.8%) or more. In addition, the surface fatiguestrength cannot be improved even if the carbide is allowed toprecipitate, unless the carbide is obtained with a content necessary forimproving the softening resistance, so that it is also necessary to makeC contained to an amount sufficient enough for the improvement.

From these points of view, the mean C concentration over the range fromthe surface of steel to a depth of 0.2 mm (also referred to as surface Cconcentration, hereinafter) is adjusted to 1.2% or more. The reason whythe range is defined from the surface of the steel to a depth of 0.2 mmis that the hardness in such range is important from the viewpoint ofthe pitting resistance. On the other hand, an excessive content resultsin production of large carbide grains, and causes insufficient hardeningproperty of the base material, thereby degrading the strength. The upperlimit of the surface C concentration is therefore limited to 3.0%.

Ratio of Carbide Area Over the Range from the Surface to a Depth of 50μm: 15% or More and 60% or Less

Precipitation of the carbide raises the surface hardness, improves thesoftening resistance over the temperature range from 200° C. to 300° C.,and improves the anti-pitting resistance. A ratio of carbide area overthe range from the surface to a depth of 50 μm of less than 15%,however, cannot fully improve the softening resistance, and cannotobtain a sufficient effect of improving the strength. On the other hand,the ratio of carbide area exceeding 60% can improve the softeningresistance, but lowers the surface fatigue and bending fatigue strength,because the carbide of a larger grain size is more likely to precipitatealong the grain boundary in a network manner. An exemplary observationof the obtained carbide is shown in FIG. 4.

Carbide Precipitated in a Finely Dispersed Manner so that the CarbideHaving a Grain Size of 10 μm or Less Accounts for 90% or More of theEntire Portion.

The carbide is a hard grain, and may serve as a starting point offatigue fracture, similarly to non-metallic inclusions such as Al oxideand Ti nitride. A smaller carbide is therefore more preferable, whereinthe grain size of which is necessarily controlled to as small as 10 μmor below, so as not to allow the carbide to exist as the starting pointof fatigue fracture. It is therefore controlled so that the carbideprecipitates in a finely dispersed manner, so that the carbide having agrain size of 10 μm or less accounts for 90% or more of the entireportion. An exemplary observation of the obtained carbide is shown inFIG. 4.

Aiming at manufacturing the above-described carburized component, amethod of manufacturing a carburized component of this inventionsubjects the steel containing the above-described steel ingredients to aprimary carburization at a temperature of Acm or above, then rapidlycools the steel to as low as point A1 or below, and then subjects thesteel to a secondary carburization at a temperature of point A1 or aboveand Acm or below. More specifically, as shown in FIGS. 1A and 1B, theprimary carburization is carried out so as not to precipitate thecarbide, at a temperature of as high as Acm or above, allowing a largesolid solubility limit of C and allowing no carbide to precipitate(between points “a” and “b”). Next, the steel is rapidly cooled so as todissolve C into a solid super-saturated manner (between points “b” and“c”). Thereafter, the steel is again heated to as high as point A1 orabove, to thereby allow fine carbide nuclei to uniformly precipitatefrom the base material super-saturated with C (between points “d” and“e”, see the upper drawing in FIG. 2), and the steel is furthersubjected to a secondary carburization so as to grow the nuclei (betweenpoints “e” and “f”, see the lower drawing in FIG. 2). Such multi-stagecarburization can realize a high-C-concentration carburization with acontrolled fine dispersion of the carbide, without allowing thenetwork-structured carbide to precipitate. In contrast to this, as shownin FIG. 3, the carburization carried out to as far as thehigh-C-concentration region before point Acm makes thenetwork-structured coarse carbide very likely to produce. Thecarburization therein is carried out by vacuum carburization (at 1,000Pa or below) as described in the above.

It is also allowable to subject the steel after the secondarycarburization to peening if necessary, to thereby further improve thestrength. Shot peening (S/P) and water jet peening (W/J), for example,are applicable to the peening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are drawings explaining carburization involved in themethod of manufacturing a carburized component of this invention;

FIG. 2 shows a schematic sectional view and a drawing of an observedsection of steel during the carburization shown in FIG. 1;

FIG. 3 shows a drawing explaining an exemplary carburization differentfrom this invention, and a drawing of an observed section; and

FIG. 4 is a drawing of an observed section of the carburized componentof this invention.

EXAMPLES

The following paragraphs will describe tests carried out for confirmingthe effects of this invention.

First, each of the steels having chemical compositions listed in Table 1was melted in a 150-kg high-frequency vacuum induction furnace. Theobtained steel ingot was rolled or hot forged so as to produce a90-mm-diameter round rod, or further hot-forged, if necessary, so as toobtain steel bar shape having a diameter of 22 to 32 mm, which was usedas a test piece.

In the compositions of comparative examples listed in Table 1, thosedeparting from the compositional ranges specified by this invention areindicated by a downward arrow (↓), for those short of the lower limits,an upward arrow (⇑), for those exceeding the upper limits. TABLE 1 C SiMn Cr Mo V Remarks Example 1 0.18 0.98 0.63 2.39 0.00 0.00 Example 20.18 0.80 0.50 2.66 0.00 0.00 Example 3 0.19 1.02 0.52 2.52 0.00 0.00Example 4 0.18 0.97 0.55 3.22 0.00 0.00 Example 5 0.18 1.48 0.55 2.580.00 0.00 Example 6 0.19 1.05 0.55 2.12 0.00 0.00 Example 7 0.19 1.080.34 2.49 0.00 0.00 Example 8 0.20 1.12 0.35 4.99 0.00 0.00 Example 90.19 0.97 0.52 2.50 0.60 0.00 Example 10 0.18 0.97 0.52 2.66 0.00 0.30Comparative ↓ 0.08  0.96 0.62 2.45 0.00 0.00 Example 1 Comparative ↑0.37  0.97 0.61 2.42 0.00 0.00 Example 2 Comparative 0.18 ↓ 0.40  0.492.92 0.00 0.00 Example 3 Comparative 0.19 ↑ 2.10  0.51 2.44 0.00 0.00Example 4 Comparative 0.18 1.10 ↓ 0.10  2.16 0.00 0.00 Example 5Comparative 0.19 0.98 ↑ 1.74  2.04 0.00 0.00 Example 6 Comparative 0.201.02 0.32 ↓ 1.10  0.00 0.00 Example 7 Comparative 0.20 1.13 0.32 ↑ 6.02 0.00 0.00 Example 8 Comparative 0.20 1.00 0.55 2.54 ↑ 1.50  0.00 Example9 Comparative 0.19 0.97 0.53 2.71 0.00 ↑ 1.50  Example 10 Comparative0.20 0.22 0.89 1.12 0.00 0.00 JIS-SCR420 Example 11

The obtained test pieces were subjected to the following evaluations.

(1) Evaluation of Manufacturability

The manufacturability was evaluated by measuring the hardness afterannealing.

A round test piece rod of 32 mm in diameter and 100 mm in length wassubjected to annealing at 920° C. for 1 hour, further annealed at 760°C. for 5 hours, and the hardness at the position of R/2 on thetransverse section was measured. The measurement of hardness conforms toJIS Z 2245 (B-scale), with a target value of HRB90 or smaller

(2) Evaluation of Basic Characteristics of Carburization

(2-1) Method of Carburization

A round test piece rod 10 mm in diameter and 100 mm in length wasfabricated, as a test piece for carburization property, from a forgedsteel bar 22 mm in diameter. The carburization was carried out in avacuum carburization furnace, using propane as the carburization gas,wherein the surface C concentration was controlled by adjusting flowrate of propane gas, diffusion time, and carburization temperature. Thecarburization was carried out at two levels of conditions so as toachieve a surface C concentration of 1.5% and 2.5%, respectively.

As for Example 3, the carburization was carried over the surface rangeof C concentration from 0.8 to 3.2%, in order to investigate influencesof the surface C concentration.

The carburization conditions are as follows.

Primary Carburization

The test piece was carburized at 1,100° C. for 70 minutes so as toadjust the C concentration to the topmost surface to about 1.2%, andthen rapidly cooled by cooling gas to a temperature range as low as 500°C. or below, to thereby allow C to intrude into the steel to at a highconcentration range so as not to be causative of precipitation of thecarbide.

Secondary Carburization

The test piece was subjected to the precipitation treatment by keepingit in the temperature range from 850° C. to 900° C., depending on thetarget carburization concentration, further carburized in thetemperature range from 850° C. to 1,000° C. for 60 to 90 minutesdepending on the target C concentration, and was hardened by immersingit into an oil bath kept at 130° C. After the hardening, the test placewas annealed at 180° C. for 120 minutes.

(2-2) Items of Evaluation

The following paragraphs will describe items of evaluation. Results ofthe evaluation are listed in Table 2. Results of Example 3 obtained byvarying the surface C concentration are listed in Table 3.

Surface C Concentration

After the carburization, C concentration was measured using a grindingchip obtained from the surface to a depth of 0.2 mm of the treated testpiece.

Ratio of Carbide Area

The transverse section of the carburized and annealed test piece rod waspolished, corroded with picral, to a portion of a depth of 50 μm fromthe topmost surface was photographed under a SEM (at a 3,000×magnification of observation), and the ratio of area was measured byimage analysis.

Size of Carbide

The test piece was observed under the same conditions as described inthe above, and the area ratio occupied by the carbide grain sized 10 μmor less was measured.

Presence or Absence of Network-Structured Carbide

The test piece was observed under the same conditions as those describedin the above, and presence or absence of the network-structured carbidewas investigated.

Presence or Absence of Incompletely-Hardened Structure

The transverse section of the carburized, annealed test piece rod waspolished, corroded with nital, to a portion of a depth of 50 μm from thetopmost surface was photographed under an optical microscope, andpresence or absence of the incompletely-hardened structure wasinvestigated.

Depth of Grain Boundary Oxide Layer

The transverse section of the carburized and annealed rod test piece waspolished, the resultant surface in an uncorroded state was observedunder an optical microscope, and the depth of the layer appearing asblack along the grain boundary at the topmost surface was measured.

Temper Softening Resistance

The carburized and annealed test piece rod was further annealed at 300°C. for 180 minutes, the transverse section was polished, and thehardness at a depth of 50 μm from the topmost surface was measured. Thehardness herein conforms to JIS Z 2244 (Hv0.3), wherein a value of Hv750or above is considered as an index ensuring a sufficient effect ofimproving the strength (≧30%: in comparison with SCR420 gas eutecticcarburized steel). TABLE 2 Carburization (1) [targeted at 1.5% C.] Ratioof Ratio of area Incompletely- Depth of grain Anneal Surface C carbideof grains Network- hardened boundary oxide 300° C. temper hardnessconcentration area ≦10 μm structured carbide structure layer hardnessExample 1 83 1.62 21% 100% no no no 775 Example 2 81 1.62 20% 93% no nono 751 Example 3 84 1.51 19% 100% no no no 779 Example 4 84 1.53 21%100% no no no 787 Example 5 89 1.41 16% 100% no no no 760 Example 6 841.65 21% 100% no no no 773 Example 7 83 1.61 23% 99% no no no 780Example 8 88 1.75 29% 94% no no no 807 Example 9 90 1.65 24% 100% no nono 802 Example 10 90 1.61 23% 100% no no no 800 Comparative 79 1.59 22%100% no no no 776 Example 1 Comparative 92 1.61 22% 100% no no no 776Example 2 Comparative 76 1.65 22% 45% yes no no 729 Example 3Comparative 97 1.31 13% 100% no no no 734 Example 4 Comparative 80 1.6220% 100% no no no 735 Example 5 Comparative 93 1.64 23% 100% no no no779 Example 6 Comparative 80 1.51 20% 100% no yes no 743 Example 7Comparative 91 1.83 32% 91% no no no 814 Example 8 Comparative 99 1.6926% 94% no no no 814 Example 9 Comparative 100 1.68 25% 100% no no no816 Example 10 Carburization (2) [targeted at 2.5% C.] Ratio of Ratio ofarea Network- Incompletely- Depth of grain 300° C. Surface C carbide ofgrains structured hardened boundary oxide temper concentration area ≦10μm carbide structure layer hardness Remarks Example 1 2.53 40% 97% no nono 824 Example 2 2.52 52% 93% no no no 835 Example 3 2.45 49% 95% no nono 843 Example 4 2.64 55% 98% no no no 863 Example 5 2.31 35% 94% no nono 828 Example 6 2.47 37% 97% no no no 817 Example 7 2.74 52% 94% no nono 845 Example 8 2.83 58% 92% no no no 875 Example 9 2.56 52% 98% no nono 861 Example 10 2.58 50% 99% no no no 845 Comparative 2.48 41% 98% nono no 825 Example 1 Comparative 2.51 40% 95% no no no 825 poormachinability Example 2 Comparative 2.61 43% 36% yes no no 825 carbideshape control Example 3 failure, poor strength Comparative 1.92 27% 99%no no no 800 poor machinability, Example 4 poor carburization, poorstrength Comparative 2.46 38% 98% no yes no 778 poor hardening, Example5 poor strength Comparative 2.51 36% 98% no no no 814 poor machinabilityExample 6 Comparative 2.44 34% 95% no yes no 796 poor hardening, Example7 poor strength Comparative 2.98 66% 81% yes no no 892 poormachinability, Example 8 carbide shape control failure Comparative 2.6156% 95% no no no 875 poor machinability Example 9 Comparative 2.56 54%98% no no no 860 poor machinability Example 10

It is known from Table 2 that all of the Examples from 1 to 10 raise noproblem in the manufacturability (anneal hardness≦HRB90), show noincomplete-hardened structure, network-structured carbide and grainboundary oxidation causative of degradation in the hardness, and givesufficient levels of temper hardness (≧750Hv) at 300° C. In contrast,Comparative Examples 2, 4, 6 and 8 to 10 show large hardness afterannealing, and raise a problem in the manufacturability. ComparativeExamples 3 and 8 show only insufficient control levels of finedispersion of the carbide due to a low Si and a large Cr content, andproduction of the network-structured carbide and other coarse carbidemay undesirably degrade the strength. Comparative Example 4, too largein the Si content, raises a problem in the manufacturability, inhibitsthe carburization property, and cannot allow the carburization toproceed to a sufficient degree. Comparative Examples 5 and 7, low in theCr and Mn contents, which give only poor levels of hardening property,show the incompletely-hardened structure, and may undesirably degradethe strength. TABLE 3 Surface C Ratio of Ratio of area Network- C Si MnCr Mo V concentration carbide area of grains ≦10 μm structured carbideExample 3 0.19 1.02 0.52 2.52 0.00 0.00 ↓ 0.80  ↓ 0%  ↓ 0% no ↓ 1.11  ↓12%  100% no 1.51 19% 100% no 1.99 34% 100% no 2.45 47%  95% no ↑ 3.15 ↑ 67%  ↓ 64%  yes Incompletely- Depth of grain 300° C. temper Ratio ofsurface hardened structure boundary oxide layer hardness fatiguestrength Remarks Example 3 no no 644 1.09 poor strength no no 734 1.23poor strength no no 779 1.43 no no 816 1.45 no no 843 1.55 no no 8941.28 carbide shape Failure, poor strength

It is known from Table 3 that the carburization targeted at a surface Cconcentration of less than 1.2% is successful in improving the surfacefatigue strength, but unsuccessful in obtaining a sufficient effect forimproving the strength (≧30%). On the other hand, the carburizationtargeted at a surface concentration exceeding 3.0% is successful inobtaining a sufficient level of 300° C. temper hardness, but shows thenetwork-structured carbide and coarse carbide, and is unsuccessful inobtaining a sufficient effect for improving the strength.

(3) Evaluation of Surface Fatigue Strength

The surface fatigue strength was evaluated using a roller pittingtester, wherein the surface fatigue strength was defined as the pressureon the load surface not causative of pitting over 107 cycles of thetest. More specifically, a 32-mm-diameter round rod was softened bykeeping it heated at 950° C., followed by gradual cooling, and was thenmachined to fabricate a roller pitting test piece having a diameter oftest portion of 26 mm. A roller correspondent to the test piece wasconfigured using SUJ2, and subjected to quench-and-temper so as toattain a hardness of HRC61. The radii of curvature of large rollers are150R and 700R.

The carburization was simultaneously carried out with the carburizationcarried out for basic evaluation of the inventive steel. A portion ofthe roller pitting test piece after the carburization was tempered at300° C. for 3 hours, and evaluation was also made on the carbonconcentration, ratio of the carbide area, the maximum carbide size andtemper hardness. The surface fatigue strength of each material wasexpressed by an index, assuming the surface fatigue strength ofgas-eutectic-carburized JIS-SCR420 material is 1.0. A sufficient effectof improving the strength by 30% or more as compared withgas-eutectic-carburized JIS-SCR420H steel was targeted.

Results of the evaluation are listed in Table 4. TABLE 4 Roller pittingtest Incompletely- Depth of grain Surface C Ratio of Ratio of areaNetwork- hardened boundary oxide concentration carbide area of grains≦10 μm structured carbide structure layer Example 1 2.04 33% 100% no nono Example 2 2.13 30% 98% no no no Example 3 1.99 34% 100% no no noExample 4 2.08 40% 100% no no no Example 5 1.94 31% 97% no no no Example6 2.00 32% 98% no no no Example 7 2.34 33% 94% no no no Example 8 2.4946% 94% no no no Example 9 2.15 34% 100% no no no Example 10 2.13 37%99% no no no Comparative Example 1 2.01 32% 100% no no no ComparativeExample 2 2.03 33% 100% no no no Comparative Example 3 2.20 29% 69% yesno no Comparative Example 4 1.83 24% 100% no no no Comparative Example 52.01 30% 96% no yes no Comparative Example 6 1.95 29% 98% no no noComparative Example 7 2.10 30% 97% no yes no Comparative Example 8 2.6154% 88% yes no no Comparative Example 9 2.19 39% 98% no no noComparative Example 10 2.12 42% 99% no no no Comparative Example 11 0.780% 0% 0% no 8 μm Roller pitting test 300° C. temper Surface fatiguehardness strength Remarks Example 1 805 1.44 Example 2 798 1.40 Example3 816 1.45 Example 4 825 1.51 Example 5 800 1.49 Example 6 804 1.46Example 7 813 1.43 Example 8 841 1.54 Example 9 832 1.52 Example 10 8231.49 Comparative Example 1 806 0.93 poor strength of core portionComparative Example 2 805 1.46 poor machinability Comparative Example 3782 1.17 carbide shape control failure, poor strength ComparativeExample 4 781 1.47 poor machnability, poor carburization ComparativeExample 5 766 1.15 poor hardening, poor strength Comparative Example 6796 1.41 poor machinability Comparative Example 7 769 1.18 poorhardening, poor strength Comparative Example 8 861 1.29 poormachinability, carbide shape control failure Comparative Example 9 8451.57 poor machinability Comparative Example 10 838 1.53 poormachinability Comparative Example 11 620 1.00 JIS-SCR20 (base steel)-gascarburizaton

It is known from Table 4 that all of the Examples from 1 to 10 aresuccessful in obtaining sufficient levels (≧30%) of improvement in thestrength. In contrast, Comparative Example 1 show only a low strengthdue to poor strength of the core portion. Comparative Examples 2, 4, 6,9 and 10 are successful in sufficiently improving the strength, butraise a problem in the manufacturability. Comparative Examples 3 and 8show growth of the network-structured carbide and other coarse carbide,and fail in obtaining sufficient levels of effect for improving thestrength. Comparative Examples 5 and 7, having low contents of Cr andMn, show only poor hardening properties as indicated by theincompletely-hardened structure, and fail in obtaining sufficient levelsof effect for improving the strength.

As proven by the above-described tests, the carburized component of thisinvention was confirmed as having a large amount of fine carbide grainsprecipitated in the surficial portion thereof, as being substantiallyfree from the grain boundary oxide layer in the surficial portion, andbeing excellent in the areas of surface hardness and strength.

FIG. 1A

primary carburization

secondary carburization

FIG. 1B, FIG. 3

γ single phase

FIG. 2

between d-e

precipitation of fine carbide grains

between e-f

growth of carbide grains

1. A carburized component consisting essentially of, in % by mass andboth ends inclusive, C: 0.1-0.30%, Si: 0.80-1.50%, Mn: 0.30-1.20%, Cr:2.0-5.5%, and the balance of Fe and inevitable impurities; having a meanC concentration over the range from the surface of the steel to a depthof 0.2 mm after vacuum carburization of 1.2% or more and 3.0% or less,having a ratio of carbide area over the range from the surface to adepth of 50 μm of 15% or more and 60% or less, having the carbideprecipitated in a finely dispersed manner so that the carbide having agrain size of 10 μm or less accounts for 90% or more of the entireportion, and having a depth of a grain boundary oxide layer of 1 μm orless.
 2. The carburized component as claimed in claim 1, furthercomprising either or both of Mo: 0.2 to 1.0% and V: 0.2 to 1.0%.
 3. Amethod of manufacturing a carburized component described in claim 1,subjecting steel containing the above-described steel ingredients to aprimary carburization at a temperature of Acm or above, then rapidlycooling the steel to as low as point A1 or below, and then subjectingthe steel to a secondary carburization at a temperature of point A1 orabove and or Acm below.
 4. The method of manufacturing a carburizedcomponent as claimed in claim 3, wherein the carburization is carriedout by vacuum carburization at 1,000 Pa or below.
 5. The method ofmanufacturing a carburized component as claimed in claim 3, furthersubjecting the steel to peening after the secondary carburization.